This invention relates to the control of an internal combustion engine in response to combustion pressure and particularly to the control of engine ignition timing. The development of piezoelectric pressure sensors suitable for combustion pressure sensing and microcomputers suitable for internal engine control has created a great deal of interest in engine control in response to combustion pressure, since it is recognized that a great deal of useful information about the combustion process is derivable from the combustion pressure curve.
One approach to the analysis of a combustion pressure curve is the examination of its harmonics. An approximation of a typical combustion pressure waveform and its harmonics at the engine firing frequency, twice this frequency and four times this frequency are shown in FIGS. 5b, 5c, 5d and 5e, respectively. I have discovered certain relationships between these harmonics that show promise in engine control. One such relationship involves the phase difference of corresponding peaks of two different harmonics of the combustion pressure curve. At MBT or optimum ignition timing, these peaks line up in phase. When timing is too retarded or advanced, these peaks shift relatively in phase in a predictable way, as indicated by the vertical broken lines and horizontal arrows, so that a closed loop control of ignition timing is possible by detecting the phase difference in the corresponding peaks of the two harmonics.